Getting hitched and having children causes mens' testosterone levels to drop, new research has shown.
Scientists have known for some time that having a high level of testosterone to start with is a recipe for reproductive success. But at the same time, men who are fathers tend to show lower levels of testosterone when tested. So, do the testosterone-charged see their hormone levels fall with fatherhood, or are men with lower levels of testosterone to start with actually more likely to become fathers overall?
To find out, Northwestern University scientist Lee Gettler and his colleagues followed over 600 Philippino men, at the ages of 21 and 26, assessing their morning and evening testosterone levels as well as social factors like marriage and fatherhood.
As expected, men with higher testosterone levels to start with were more likely to have married and had children during the course of the study compared with participants with lower testosterone levels. But these same married fathers subsequently showed the greatest declines in testosterone levels, of up to 34% below the median, by age 26 compared with their readings at age 21. This was even after controlling for factors that might potentially bias the findings, like sleep deprivation caused by crying babies.
Men who remained single and had no children during the study, on the other hand, showed more modest reductions of only 12% below the median across the same period.
Fathers who spent more than 3 hours per day caring for their children also had significantly lower levels of testosterone compared with the less-involved fathers. This is almost certainly an evolved trait, the authors argue in their paper in PNAS. This is because, although testosterone-fuelled aggression and a tendency towards competitiveness are useful for winning a mate in the first place, these behaviours are much less useful when it comes to caring for kids subsequently. Instead, lower testosterone levels lend themselves to a more nurturing instinct and reduced rates of marital disharmony.
The findings therefore turn some accepted views of anthropology and evolution on their heads. The fact that men can adjust their hormonal thermostats when they adopt a care-giving role argues that there's more to males than just hunting and sex.
At the moment the mechanism underlying the effect isn't clear. It could have some other hidden health benefits, however, including a lower risk of prostate cancer, which can be triggered by testosterone, lower levels of cholesterol and less risk-prone lifestyle. Bring on the babies...